How to make deception by NPCs work in an engaging manner

Question

We are playing in a world with a certain degree of realism. For me, this includes deception.

Deception can be found anywhere:

1. Making a small and probably inconsequential lie to make yourself look better.
2. Hiding information in order to gain an advantage (sell a bad object for a higher price than its value, pay less for a service, etc.)
3. Actively lying to someone for ill intentions such as trying to rob them.

I think it's normal for NPCs to not always tell the truth in a conversation with a PC. This does not mean that everyone is always lying. But it means that the truth gets bent at times for reasons such as the ones listed above.

I want to give the players a chance to work around this. However, I am afraid that they'll start losing trust in everything they see. A lot of conversations have started to include OOC phrases such as:

'Also, I want to check for the true intentions. Can I roll for Insight?'

and similar. While that's totally fine, according to my understanding of all rules, it totally ruins the roleplaying atmosphere. Whenever something is said, a roll follows and phrases like this have to follow:

'You can not see a lie in their words.'

'The pure fear in their eyes leads you to think that they are telling the truth.'

'They are quite nervous. Could they be hiding something?'

Those phrases start to repeat themselves a lot as I can only find so many ways to say "they are telling the truth".

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My question is: How can I make NPCs use Deception without the players starting to question every single thing they hear from anyone around the world, leading to a slow evening with ruined roleplay?

Note: While this question is tagged for DND-5e and specifically mentions the Insight skill, this could probably be generalized for other games as well.

Answer 1

Passive Checks!

While Passive Perception is the most common, the rules for Passive Checks indicate one can use it for any skill one likes¹. Situations like you describe would be well served by Passive Insight - it allows one to keep the rolling entirely on the DM's side, but still takes the character's abilities into account.

Who Calls For Rolls

In social scenes, players should never ask for rolls. The DM describes the scene, the players describe their character's actions, and the DM determines if any rolling is needed. (In a combat scene, the opposite is somewhat true - the player should know what they need to roll for their attacks, or what they need to ask the DM to roll for saves.)

If the character has reason to be suspicious, the player should say so, describing why. The DM can then ask for an Insight roll, or go with the passive approach described above. The passive method has some advantages, preventing a player from metagaming based on the die roll - if the DM says an NPC is telling the truth, but the player rolled low, some players can't help but decide the DM is lying.

But They're Paranoid!

If the table is full of players with characters who are paranoid and constantly suspicious, you have a different issue. As the DM, you have to assure your players (and get them to believe) that you will call for rolls when appropriate. You need to convince them to assume basic truthfulness, rather than assuming deceit.

This is another place that makes the passive approach even more valuable. Of course, in order to avoid telegraphing the possibility of Deception, the DM will need to make unnecessary (and ultimately discarded) rolls behind the scenes. Handling it via Passive Checks saves time; the DM doesn't have to wait for each player to roll and calculate a result, they can just roll some dice, make appropriate noises, and keep the scene flowing.

¹_{Personally, I'm a big fan of Passive Insight and Passive Investigation.}

Answer 2

Stop using Insight as a lie detector test.

Instead of using Insight as a lie detector test, I let the players decide whether or not an NPC is lying. I find that doing so increases and rewards player engagement, because they have to pay a lot more attention to the game in general.

As you point out, players trigger Insight checks all the time because it's a cost-free lie detector test. However, bad rolls often lead to a sort of cognitive dissonance where players either try too hard not to metagame or take the opposite of what you tell them. Generally, it's very difficult to dictate how a character is feeling to the player that's playing them.

Instead, players must use their existing knowledge and skills to catch lies. After all, in real life, most lies are caught because they create inconsistencies. For example, I might call for a knowledge-related check to see if the PCs know that the wizard is lying about how a spell works. For your examples, the inconsequential lies would probably slide, but a character could roll an INT check for appraising the fake item, or a perception check to notice the ambush ahead. If a player isn't paying enough attention, then they'll miss out on important clues. If it's been a long time, or it's something their character would have noticed, sometimes I remind the player of something or drop hints at the table.

If a player really wants to use insight and rolls well, I don't tell them "you know he's lying" or "he's not lying". Instead, I give the players information based on their body language or tone, such as "he's hunched over and looking shifty" or "he sounds really confident". After all, the basis for insight's lie detection is just body language (PHB 178):

Doing so involves gleaning clues from body language, speech habits, and changes in mannerisms.

This also leaves space for real lie detectors, like Detect Thoughts or Zone of Truth.

One of the drawbacks of this method is that players might be more or less wise than their characters. Still, I find that the engagement benefits of shifting the burden of lie detection from the characters to the players outweighs the potential "out of character" behavior. Inconsequential lies will often stand, as they often do, but big lies have a greater chance of being found out.

Answer 3

Go Beyond Insight

Insight is based on direct observation of a creature. But other skills can be used to indirectly gauge an NPC. You could use passive checks against any of these skills to provide clues to the PCs without making it feel like a series of direct truthfulness checks:

• Investigation: What about the NPC is at odds with the way she described herself and her intentions? She claims to be a hunter but her boots are hard-soled and her cloak is too flashy.
• Nature: She described spending all summer hunting in the Arcadian Forest, but you know that area is devoid of large game that time of year because of peculiar growth cycle of the longspear grasses that cover the forest floor.
• Religion: Most hunters follow the wood goddess Clymetra, but you don't see any of the greenish-yellow half-moon symbols most of her followers wear. This isn't proof of a lie, but it's enough to make a PC suspicious.
• Perception: She's talking to you in a crowded tavern. Some of the local farmers are looking in your direction with grins on their faces. Is that because they've seen her running similar scams on travelers in the past?

There are doubtless other skill checks you could use to provide more nuanced evaluations of NPC intentions and behavior.